Since macrophages are considered to be major effector cells in host defense against tumors, increasing their concentration at a tumor site should be of therapeutic value. Elevation of the numbers of macrophages in guinea pig hepatomas has been achieved by the in vivo administration of covalent conjugates of IgG antibodies reactive with tumor cell surface antigens and the chemotactic peptide, formylmethionylleucylphenylalanine (fMLP). These conjugates, which are chemotactic for guinea pig macrophages in vitro and bound to tumor cells but not to normal liver cells, fibroblasts or fibrosarcoma cells, significantly (p less than .005) increased the numbers of macrophages in the tumors when administered either in a single dose or in five doses. Although five injections of unconjugated fMPL were nearly as effective as the conjugates, free fMPL did not enhance the numbers of macrophages in tumors when injected as a single dose. Unconjugated IgG was ineffective. The mean tumor weights were decreased in those groups of guinea pigs which received the conjugates but statistical significane was not achieved due to tumor weight variability in all groups/